Conventional silk screen presses print multi-colored images on material by mounting the material to a platen and rotating the platen past each of a plurality of print units, located peripherally about a central support, wherein each print unit prints a different color. The images printed at the several print units, when superimposed one over the other on the material, produce the desired multi-colored work. It is important in multi-colored apparatus of this type to completely cure, or dry, the ink applied at a previous station prior to application of a differently colored ink at a subsequent station. Curing between successive ink applications is sometimes necessary or desirable in order to avoid smearing or blurring of the previously printed image upon printing of a subsequent image thereupon. It is known to utilize electrical resistance heating elements situated in proximity with the printed material, or workpiece, which impart radiative and convective heat to the printed material between successive printing operations sufficient to cure the print thereon. Since only a thin layer of ink is applied in silk screening applications, the required heat exposure time for curing is relatively short. Overheating of the printed work may result in wrinkles, discoloration, shrinkage, and/or scorching of both the applied ink and the underlying material. Therefore, it is important that the heat application be closely controlled.
Normally, only a thin layer of ink is applied in screen printing operations, and exposure to ambient air is sufficient to adequately cure or dry the applied ink. However, certain applications require a heavier layer of applied ink. For instance, before printing fluorescent ink upon a black material such as a T-shirt, it is necessary to apply a heavy layer of white ink to completely cover up the black substrate. Thus, curing is generally necessary immediately following such a white layer print unit at which the heavy layer of white ink is applied.
The heat generated by electrical resistance heaters increases with time upon application of a given voltage, so that the heating elements require time to reach their point of maximum heat emission. A particular problem with current designs is that they are not able to reconcile the conflicting goals of providing maximum heating during curing and interruption of heat to the printed material between curing operations, without significantly reducing production speed. Two alternative methods are employed in current curing apparatus. Either the heating elements are maintained at a constant high heating level so that no time is lost in bringing the heating elements back up to their maximum level, or else the voltage supplied to the heating elements is completely interrupted between curing operations and reapplied during curing. Both designs have been found to be inadequate. Maintaining the heating elements at high voltage has been found to cause high heat build-up, and if the apparatus is stopped from indexing the workpiece, a paper or textile workpiece can be subjected to sufficient heat to scorch or to catch fire. To prevent scorching or burning of the material when the indexing movement is stopped, the voltage may be interrupted. However, interruption of the voltage to the heating elements between curing operations is undesirable in that production speeds are limited by the time required for the heating elements to reattain their maximum heating level upon reapplication of electrical power thereto.
In use, the curing elements are usually set at their maximum full power position during start-up to heat the curing elements to their operating temperature which can be varied substantially by operation of a power control means such as a manually operable rheostat. For example, when the power is set at full power, i.e., 100% power, the maximum temperature achieved is on the order of 380.degree. F. to 400.degree. F. within thirty or more seconds. Usually, the operator will adjust the rheostat to a percentage, e.g., 50% of full power, to operate at a lower temperature, for example, 250.degree. F., to accommodate for various parameters such as the particular inks being cured, the kind of workpiece being printed, and ambient conditions. The exact settings of power are based on trial and error and experience. Whenever there is an interruption in the usual printing cycle, the power is interrupted completely or reset to a very low level of power, e.g., 25% of full power, to prevent damage to the workpiece in the curing station. After the interruption is over, the operator usually turns the rheostat to full power so that the heating elements will heat more quickly to a temperature needed to cure the ink. The operator will, after a short interval, return the rheostat to the previous setting and begin cycling the screen printing machine. If the operator is not careful and forgets to return the rheostat to the desired lower percentage of power, the workpieces may be damaged by the too high temperature curing conditions until the operator recognizes his mistake.
In co-pending application, Ser. No. 773,486, entitled "Resin Curing apparatus and Method Utilizing Infrared Lamp and Blower Control Means", filed Oct. 9, 1991, there is disclosed and claimed apparatus in which the voltage supplied to the heating elements during the intervals between curing operations is dropped to a fraction of the curing voltage, and a thin layer of high velocity air is simultaneously blown between the heating elements and the workpiece. The airflow rate is sufficient to dissipate the low heat generated by the heating elements at the reduced voltage, and any residual heat from the heating elements and their housing, away from the workpiece. The airflow also increases convection at the workpiece surface, which further assists in the cooling thereof between curing operations. While a predetermined full voltage sufficient to effect curing is supplied to the heating elements during curing, the voltage is reduced to approximately one quarter of full voltage between curing operations and during production interruptions and, during such periods of low voltage, air is blown between the apparatus and printed material. Upon resumption of normal operation, full voltage is resumed and the airflow discontinued.
In the apparatus disclosed in the above cited application, the supplied voltage is alternated between high and low levels, and the blower turned on and off, based upon signals sent from a programmable control panel. Quartz tubes or similar electrical resistance heating elements are employed which allow operation under both partial and full voltages with the heat generated by the tubes proportional to the applied voltage. During periods of production interruption, a reduced voltage continues to be supplied to the heating elements. This allows the heating elements to reattain their maximum heating level more rapidly upon reimposition of full voltage than designs wherein the voltage to the heating elements is completely interrupted between curing operations. This apparatus may also be supplied with a rheostat or other power control device that can be set a fraction of full power for normal curing and then manually reset to full power to heat the quartz tubes rapidly. The operator must then remember to reset the rheostat to the desired percentage of full power to avoid damage to the workpieces.
Although the apparatus disclosed in the above cited application solves many then-existing problems in curing workpieces in screen printing presses, the lowering of the voltage during production interruptions and operating the heating elements at maximum voltage during curing operations presents some problems in controlling the uniformity of the curing operation and adapting to varying needs for different production processes.